For many wireless network applications, such as a WVoIP (Wireless Voice over Internet Protocol) system, power saving (PS) of voice clients and quality of Service (QoS) are major concerns. Power savings is a concern because wireless clients are usually battery powered; the more power that is saved, the longer the batteries in the client last. For example, UPSD (Unscheduled Power Save Delivery) in IEEE 802.11e specification provides great power savings. Simulations have shown that UPSD can put a voice client in doze for more than 90% of time during an active talking session.
VoIP is a constant-bit rate application. VoIP packets or frames are continually generated at a constant interval, usually 10, 20 or 30 ms. Therefore, either the HCCA (Hybrid Controlled Channel Access) or the EDCA (Enhanced Distributed Channel Access) extension can be used to synchronize transfers between an AP and a station.
Using HCCA polling, once a station is accepted by an AP as a polled client, the station sleeps until the expected arrival time for a downlink poll, or poll plus VoIP frame. The station responds within a mandatory time with uplink VoIP data (or with a QoS NULL) frame. A problem with HCCA is that the poll can be delayed by many factors including but not limited to interference, a long duration frame, a scheduling conflict wherein the AP is polling another station, a previous frame exchange taking longer than expected, relative clock drift between the AP and the client, or the AP transmitting multiple downlink frames instead of the usual single frame.
The EDCA access method provides for prioritized channel access. Unlike HCCA wherein the station is constrained by the AP polling schedule, the EDCA client may operate in unscheduled automatic power save delivery (U-APSD). In this mode, the station sleeps until it has a VoIP frame ready to transmit. The AP is expecting the transmission because of prior signals exchanged between the client and the AP.
EDCA and HCCA assume the AP is configured to avoid long bursts or other behavior which may cause delays in access. Otherwise, either scheme will experience a scheduling time shift or a delay in the UPSD frame exchange.
FIG. 1 provides a graphical illustration of packet exchanges for a system implementing UPSD. The example of FIG. 1 has one AP and three clients (Clnt1, Clnt2 and Clnt3). As shown in FIG. 1, the first client, CLNT1, sends a frame, DAT1 when it wakes up, and the AP responds with an ACK (ACK1). The second client, CLNT2, then sends a frame, DAT2 and the AP responds with a subsequent ACK (ACK2). The third client, CLNT3, wakes up and sends a frame, DAT3 and the AP responds with a subsequent ACK (ACK3). The AP also sends downlink frames DAT4, DAT5, DAT6 separately to each client, CLNT1, CLNT2, CLNT3 respectively. Each client then responds to the data frames with corresponding ACKs (ACK4, ACK5 and ACK6). For the illustration in FIG. 1, there are 3 (n=3) stations, resulting in 12 frame exchanges and 6 channel access contentions. Thus, for UPSD, for a number of stations (n), there are 4n frame exchanges and 2n channel access contentions
Frame aggregation is a fundamental technology to achieve the high throughput (HT) requirement for the next generation of wireless network, covered by IEEE 802.11 TGn (Task Group n) specification. Cisco's (Cisco System, Inc., 170 West Tasman, San Jose, Calif.) contributions of MRMRA (the multi-receiver multi-response aggregation) and MRMRMRA (the multi-rate multi-receiver multi-response aggregation) to the 802.11n proposals provide a practical means to substantially improve the MAC efficiency of the network while still well maintaining the QoS (Quality of Service) by minimizing frame latencies. They are especially efficient for wireless voice over IP (WVoIP). MRMRA and MRMRMRA specify the protocols, namely the frame exchange sequences and frame formats, for multi-receiver multi-response aggregations of single rate or multi-rates. To utilize the protocols, an aggregation scheduler decides what frames to be aggregated in an aggregation at a specific time and what duration values to allocate for each of the multiple receivers.
However, as noted hereinabove, an assumption for EDCA and HCCA access is that the AP is configured to avoid long bursts or other behavior which may cause delays in access. Thus, the need exists for a system and method which allow UPSD and MRMRA to co-exist.